Al/Ga-Doped Li7La3Zr2O12 Garnets as Li-Ion Solid-State Battery Electrolytes: Atomistic Insights into Local Coordination Environments and Their Influence on 17O, 27Al, and 71Ga NMR Spectra

Li 7 La 3 Zr 2 O 12 (LLZO) garnets are among the most promising solid electrolytes for next-generation all-solid-state Li-ion battery applications due to their high stabilities and ionic conductivities. To help determine the influence of different supervalent dopants on the crystal structure and site preferences, we combine solid-state 17 O, 27 Al, and 71 Ga magic angle spinning (MAS) NMR spectroscopy and density-functional theory (DFT) calculations. DFT-based defect configuration analysis for the undoped and Al and/or Ga-doped LLZO variants uncovers an interplay between the local network of atoms and the observed NMR signals. Specifically, the two characteristic features observed in both 27 Al and 71 Ga NMR spectra result from both the deviations in the polyhedral coordination/site-symmetry within the 4-fold coordinated Li1/24d sites (rather than the doping of the other Li2/96h or La sites) and with the number of occupied adjacent Li2 sites that share oxygen atoms with these dopant sites. The sharp 27 Al and 71 Ga resonances arise from dopants located at a highly symmetric tetrahedral 24d site with four corner-sharing LiO 4 neighbors, whereas the broader features originate from highly distorted dopant sites with fewer or no immediate LiO 4 neighbors. A correlation between the size of the 27 Al/ 71 Ga quadrupolar coupling and the distortion of the doping sites (viz. XO 4 /XO 5 /XO 6 with X = {Al/Ga}) is established. 17 O MAS NMR spectra for these systems provide insights into the oxygen connectivity network: 17 O signals originating from the dopant-coordinating oxygens are resolved and used for further characterization of the microenvironments at the dopant and other sites.